Remove "support" for using malloc.c outside the perl core distribution.

Remove "support" for using malloc.c outside the perl core distribution.

It only ever worked properly in 5.005.

Commit d720c4410d3a0730 from June 1998 is the first known version of malloc.c
that would compile stand alone, without needing a config.h
Commit 5bbd1ef51ee0ebfa from December 1998 will not compile by default,
but will if one provides suitable definitions of simple macros on the command
line.
Commit 427181846486e3aa from September 1999 will not compile with command
line flags alone.

12 years later, no-one has reported problems, so it's safe to remove this.